We propose to purify the nuclear enzyme, poly (ADP-ribose) polymerase, from animal cells and to perform a detailed investigation of its physical properties, composition and substructure. Using the purified enzyme the obligatory participation of DNA and histones in the reaction will be defined by analyzing the effects of these macromolecules on enzyme structure and function. The results of this study will better define the role of poly (ADP-ribose) polymerase in the regulation of DNA synthesis and cell replication in eucaryotic cells.